1. Field of the Invention
The present invention relates to control devices for repetitive limit switching, and, more particularly, to a precision drive for adjustable rotary switching controls of a reciprocating unit, such as the injection unit of an injection molding machine.
2. Description of the Prior Art
The production of high-quality injection-molded parts requires very precise and consistantly responding limit switches for the injection unit of the injection molding machine, especially for the switching signals which start and stop the movement of the plastification screw of the injection unit. These signals determine the exact quantity of plastic material which is being injected into the injection molding die.
One prior art solution to this problem involves the use of a control device which measures the lineal displacement of the moving assembly by converting that displacement into an angular displacement of a rotatable control disc and a connected angular pulse giver whose circumferential profile is being scanned by means of an optical scanner. The lineal motion of the moving machine unit is transmitted to the control disc--which, in the case at hand, is a pinion of small diameter--by means of a precision-ground gear rack whose extremity is fixedly attached to the moving machine unit. The control disc executes several revolutions over the extent of the total lineal displacement. The result is a displacement measurement and switching action whose accuracy and reproducibility are adequate for normal requirements, though obtained with a comparatively expensive device.
Also known from the prior art is an approach which involves the use of a linear potentiometer for the measurement of the displacement of the movable machine unit and for the generation of appropriate switching pulses. One shortcoming of these devices is that linear potentiometers consist of coils which are wiped in the axial direction, with the result that the reading accuracy is less than optimal. This accuracy suffers further, when, as a result of long-run operation, wear-produced clearances affect the accuracy of guidance between the linear potentiometer coil and the wiper.